Phosphor, and radiation image storage panel

ABSTRACT

A divalent europium activated alkaline earth metal halide phosphor which is coactivated with boron and has the formula (I): 
     
         M.sup.II X.sub.2.aM.sup.II X&#39;.sub.2 :xEu.sup.2+,yB         (I) 
    
     in which M II  is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; each of X and X&#39; is at least one halogen selected from the group consisting of Cl, Br and I, and X≠X&#39;; and a, x and y are numbers satisfying the conditions of 0.1≦a≦10.0, 0&lt;x≦0.2 and 2×10 -4  ≦y≦2×10 -1 , respectively. A radiation image recording and reproducing method utilizing said phosphor and a radiation image storage panel employing said phosphor are also disclosed.

This application is a continuation of Ser. No. 047,595, filed May 11,1987, now abandoned, which itself was a continuation of Ser. No.727,974, filed Apr. 26, 1985, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a phosphor, a radiation image recordingand reproducing method utilizing the same, and a radiation image storagepanel employing the same. More particularly, the invention relates to adivalent europium activated alkaline earth metal halide stimulablephosphor, a radiation image recording and reproducing method utilizingthe same, and a radiation image storage panel employing the same.

2. Description of the Prior Art

There is well known a divalent europium activated alkaline earth metalfluorohalide phosphor (M^(II) FX:Eu²⁺, in which M^(II) is at least onealkaline earth metal selected from the group consisting of Ba, Sr andCa; and X is a halogen other than fluorine), as a divalent europiumactivated alkaline earth metal halide phosphor. The phosphor givesemission (spontaneous emission) in the near ultraviolet region whenexposed to a radiation such as X-rays. The phosphor also gives emission(stimulated emission) in the near ultraviolet region when excited withan electromagnetic wave such as visible light or infrared rays afterexposure to a radiation such as X-rays. Namely, the phoshor is astimulable phosphor.

A radiation image recording and reproducing method utilizing asitmulable phosphor can be employed in place of the conventionalradiography utilizing a combination of a radiographic film having anemulsion layer containing a photosensitive silver salt and anintensifying screen as described, for instance, in U.S. Pat. No.4,239,968. The method involves steps of causing a stimulable phosphor toabsorb a radiation having passed through an object or having radiatedfrom an object; sequentially exciting (or scanning) the phosphor with anelectromagnetic wave such as visible light or infrared rays (stimulatingrays) to release the radiation energy stored in the phosphor as lightemission (stimulated emission); photoelectrically detecting the emittedlight to obtain electric signals; and reproducing the radiation image ofthe object as a visible image from the electric signals.

In the radiation image recording and reproducing method, a radiationimage is obtainable with a sufficient amount of information by applyinga radiation to the object at a considerably smaller dose, as comparedwith the conventional radiography. Accordingly, this method is of greatvalue, especially when the method is used for medical diagnosis.

As for a stimulable phosphor employable in the radiation image recordingand reproducing method, almost no stimulable phosphor other than theabove-mentioned divalent europium activated alkaline earth metalfluorohalide phosphor has been known.

The present inventors discovered a novel divalent europium activatedalkaline earth metal halide phosphor having the following formula:

    M.sup.II X.sub.2.aM.sup.II X'.sub.2 :xEu.sup.2+

in which M^(II) is at least one alkaline earth metal selected from thegroup consisting of Ba, Sr and Ca; each of X and X' is at least onehalogen selected from the group consisting of Cl, Br and I, and X≠X';and a and x are numbers satisfying the conditions of 0.1≦a ≦10.0 and 0<x≦0.2, respectively, and applied for a patent with respect to saidphosphor, a radiation image recording and reproducing method utilizingsaid phosphor and a radiation image storage panel employing saidphosphor (U.S. patent application Ser. No. 660,987 and European patentapplication No. 84112417.5).

The novel divalent europium activated alkaline earth metal halidephosphor has been confirmed to have a crystal structure different fromthat of the aforementioned M^(II) FX:Eu²⁺ phosphor on the basis of theX-ray diffraction patterns as described in the above application. Thisphosphor gives spontaneous emission (peak wavelength: approx. 405 nm) inthe near ultraviolet to blue region when exposed to a radiation such asX-rays, ultraviolet rays and cathode rays, and also gives stimulatedemission in the near ultraviolet to blue region when excited with anelectromagnetic wave having a wavelength within the region of 450-1000nm after exposure to a radiation such as X-rays, ultraviolet rays andcathode rays. Accordingly, the phosphor is very useful as a phosphor fora radiation image storage panel employed in the radiation imagerecording and reproducing method utilizing a stimulable phosphor.

The sensitivity of the radiation image recording and reproducing methodemploying the radiation image storage panel to a radiation generallyincreases as the luminance of stimulated emission of the phosphoremployed therefor increases. Accordingly, the stimulable phosphoremployed for the panel is desired to show the stimulated emission ofluminance as high as possible.

SUMMARY OF THE INVENTION

A principal object of the present invention is to provide a phosphorimproved in the luminance of stimulated emission with respect to theabove-mentioned novel divalent europium activated alkaline earth metalhalide phosphor.

Another object of the present invention is to provide a radiation imagerecording and reproducing method improved in the sensitivity and aradiation image storage panel employed therein.

As the results of the study on the novel divalent europium activatedalkaline earth metal halide phosphor, the present inventors found that aphosphor obtained by coactivating said phosphor with a specific amont ofboron shows stimulated emission of high luminance, and accomplished theinvention.

The phosphor of the invention is a divalent europium activated alkalineearth metal halide phosphor which is coactivated with boron and has theformula (I):

    M.sup.II X.sub.2.aM.sup.II X'.sub.2 :xEu.sup.2+, yB        (I)

in which M^(II) is at least one alkaline earth metal selected from thegroup consisting of Ba, Sr and Ca; each of X and X' is at least onehalogen selected from the group consisting of Cl, Br and I, and X≠X';and a, x and y are numbers satisfying the conditions of 0.1≦a ≦10.0,0<x≦0.2 and 2×10⁻⁴ ≦y≦2×10⁻¹, respectively.

The present invention further provides a radiation image recording andreproducing method utilizing the above phosphor and a radiation imagestorage panel using said phosphor.

That is, the radiation image recording and reproducing method comprisessteps of:

(i) causing the divalent europium activated alkaline earth metal halidephosphor which is coactivated with boron and has the formula (I) toabsorb a radiation having passed through an object or having radiatedfrom an object;

(ii) exposing said stimulable phosphor to an electromagnetic wave havinga wavelength within the range of 450-1000 nm to release the radiationenergy stored therein as light emission; and

(iii) detecting the emitted light.

The radiation image storage panel of the invention comprises a supportand a stimulable phosphor layer provided thereon, in which saidstimulable phosphor layer contains the divalent europium activatedalkaline earth metal halide phosphor which is coactivated with boron andhas the formula (I).

The phosphor of the invention having the formula (I) obtained bycoactivating the above-mentioned novel divalent europium activatedalkaline earth metal halide phosphor with a specific amount of boron ishighly enhanced in the luminance of stimulated emission when excitedwith an electromagnetic wave having a wavelength within the range of450-1000 nm after exposure to a radiation such as X-rays. Further, theradiation image recording and reproducing method of the inventionutilizing the phosphor having the formula (I) is remarkably improved inthe sensitivity, and the radiation image storage panel employing saidphosphor has the highly enhanced sensitivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a relationship between y value and luminance of stimulatedemission with respect to BaCl₂.BaBr₂ :0.01Eu²⁺, yB phosphor, which is anexample of the divalent europium activated alkaline earth metal halidephosphor coactivated with boron of the invention.

FIG. 2 is a schematic view showing the radiation image recording andreproducing method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The divalent europium activated alkaline earth metal halide phosphor ofthe present invention which is coactivated with boron can be prepared,for instance, by a process described below.

As starting materials, the following materials can be employed:

(1) at least two alkaline earth metal halides selected from the groupconsisting of BaCl₂, SrCl₂, CaCl₂, BaBr₂, SrBr₂, CaBr₂, BaI₂, SrI₂ andCaI₂ ;

(2) at least one compound selected from the group consisting of boroncompounds such as boron halide and boron oxide; and

(3) at least one compound selected from the group consisting of europiumcompounds such as europium halide, europium oxide, europium nitrate andeuropium sulfate.

As the starting material (1), two or more kinds of alkaline earth metalhalides having a halogen different from each other are employed.Further, ammonium halide (NH₄ X", in which X" is any one of Cl, Br andI) may be employed as a flux.

The alkaline earth metal halides (1), boron compound (2) and europiumcompound (3) are, in the first place, mixed in the stoichiometric ratiocorresponding to the formula (II):

    M.sup.II X.sub.2.aM.sup.II X'.sub.2 :xEu,yB                (II)

in which M^(II), X, X', a, x and y have the same meanings as definedabove.

The mixture of starting materials for the phosphor of the presentinvention is prepared by any one of the following procedures:

(i) simply mixing the starting materials (1), (2) and (3);

(ii) mixing the starting materials (1) and (2), heating the obtainedmixture at a temperature of not lower than 100° C. for several hours andthen mixing the heat-treated mixture with the starting material (3); and

(iii) mixing the starting materials (1) and (2) in the form of asolution, drying the solution by reduced pressure drying, vacuum dryingor spray drying under heating (preferably, 50°-200° C.), and then mixingthe obtained dry product with the starting material (3).

Further, as a modification of the procedure (ii), there may be mentioneda procedure comprising mixing the starting materials (1), (2) and (3)and subjecting the obtained mixture to the heating treatment; or aprocedure comprising mixing the starting materials (1) and (3),subjecting the obtained mixture to the heating treatment and mixing thestarting material (2) with the heat-treated product. As othermodification of the procedure (iii), there may be mentioned a procedurecomprising mixing the starting materials (1), (2) and (3) in the form ofa solution and subjecting the solution to the drying; or a procedurecomprising mixing the starting materials (1) and (3) in the form of asolution, subjecting the solution to the drying and mixing the obtaineddry product with the starting material (2).

The mixing is carried out using a conventional mixing apparatus such asa variety of mixers, a V-type blender, a ball mill and a rod mill in anycase of the procedures (i), (ii) and (iii).

Then, the resulting mixture of the starting materials is placed in aheat-resistant container such as a quartz boat, an alumina crucible or aquartz crucible, and fired in an electric furnace. The temperature forthe firing suitably ranges from 500° to 1300° C., and preferably rangesfrom 700° to 1000° C. The firing period is determined depending upon theamount of the mixture of starting materials, the firing temperature,etc., and suitably ranges from 0.5 to 6 hours. As the firing atmosphere,there can be employed a weak reducing atmosphere such as a nitrogen gasatmosphere containing a small amount of hydrogen gas or a carbon dioxidegas atmosphere containing carbon monoxide gas. A trivalent europiumcompound is generally employed as the above-mentioned starting material(3) and in the firing stage, the trivalent europium contained in themixture is reduced into divalent europium by the weak reducingatmosphere.

After firing the mixture of starting materials for the phosphor asdescribed above, the fired product is taken out of the furnace, allowedto stand for cooling and pulverized. The pulverized product may be againfired (second firing). As the firing atmosphere for the second firing,there can be employed a neutral atmosphere such as a nitrogen gasatmosphere and an argon gas atmosphere as well as the above-mentionedweak reducing atmosphere. The temperature for the second firing rangesfrom 500° to 800° C., and the firing period ranges from 0.5 to 12 hours.

Through the firing procedure, a phosphor of the present invention isproduced. The phosphor thus obtained may be processed in a conventionalmanner involving a variety of procedures for the preparation ofphosphors such as a washing procedure, a drying procedure and a sievingprocedure.

The phosphor of the present invention prepared in accordance with theabove-described process is a divalent europium activated alkaline earthmetal halide phosphor which is coactivated with boron and has theformula (I):

    M.sup.II X.sub.2.aM.sup.II X'.sub.2 :xEu.sup.2+,yB         (I)

in which M^(II) is at least one alkaline earth metal selected from thegroup consisting of Ba, Sr and Ca; each of X and X' is at least onehalogen selected from the group consisting of Cl, Br and I, and X≠X';and a, x and y are numbers satisfying the conditions of 0.1≦a≦10.0,0<x≦0.2 and 2×10⁻⁴ ≦y≦2×10⁻¹, respectively.

With respect to the phosphor of the invention having the formula (I),the number for y indicating the amount of boron activator is preferablywithin the range of 4×10⁻⁴ ≦y≦10⁻¹ from the viewpoint of enhancement inthe luminance of stimulated emission. From the same viewpoint, thenumber for a indicating the ratio between M^(II) X₂ and M^(II) X'₂ ispreferably within the range of 0.3≦a≦3.3 and more preferably of0.5≦a≦2.0, and the number for x indicating the amount of europiumactivator is preferably within the range of 10⁻⁵ ≦x≦10⁻¹.

An example of the phosphor having the formula (I), BaCl₂.BaBr₂:0.001Eu²⁺,yB phosphor, has a relationship between y value (the amountof boron) and the luminance of stimulated emission as shown in FIG. 1.

FIG. 1 graphically shows a relationship between y value and luminance ofstimulated emission [emission luminance upon excitation with asemiconductor laser beam (wavelength: 780 nm) after exposure to X-raysat 80 KVp] with respect to BaCl₂.BaBr₂ :0.001Eu²⁺,yB phosphor. As isevident from FIG. 1, the BaCl₂.BaBr₂ :0.001Eu²⁺,yB phosphor having yvalue within a range of 2×10⁻⁴ ≦y≦2×10⁻¹ gives stimulated emission ofhigher luminance than the phosphor containing no boron (y=0). On thebasis of this fact, the y value range of the divalent europium activatedalkaline earth metal halide phosphor of the invention which iscoactivated with boron, namely 2×10⁻⁴ ≦y≦2×10⁻¹, has been decided. As isalso evident from FIG. 1, the phosphor having y value within a range of4×10⁻⁴ ≦y≦10⁻¹ gives stimulated emission of prominently high luminance.

It has been confirmed that phosphors according to the present inventionand have M^(II), X, X', a and x other than the above-stated one have thesame tendencies in the relationships between y value and the luminanceof stimulated emission as shown in FIG. 1.

The phosphor of the invention may contain other various additives as faras the effect given by adding boron thereto (enhancement in theluminance of stimulated emission) is not reduced.

The above-described divalent europium activated alkaline earth metalhalide phosphor which is coactivated with boron and has the formula (I)shows a stimulation spectrum similar to that of the divalent europiumactivated alkaline earth metal halide phosphor as illustrated in theaforementioned U.S. patent application Ser. No. 660,987. Since thewavelength region of the stimulation spectrum is wide and 450-1000 nm,it is possible to vary the wavelength of stimulating rays for excitingthe phosphor in the radiation image recording and reproducing method ofthe invention. It means that a source of stimulating rays can besuitably selected according to the purpose. For example, a semiconductorlaser (having a wavelength in the infrared region) which is in a smallsize and needs only weak driving power can be employed as the source ofstimulating rays, and accordingly the system for carrying out the methodcan be made compact. From the viewpoint of the luminance of stimulatedemission and of the separation on wavelength between the emitted lightand stimulating rays, the stimulating rays are preferred to be anelectromagnetic wave having a wavelength within the range of 500-850 nm.

The divalent europium activated alkaline earth metal halide phosphorcoactivated with boron and having the formula (I) is preferably employedin the form of a radiation image storage panel (also referred to as astimulable phosphor sheet) in the radiation image recording andreproducing method of the invention.

The radiation image storage panel comprises a support and at least onephosphor layer provided on one surface of the support. The phosphorlayer comprises a binder and a stimulable phosphor dispersed therein.Further, a transparent protective film is generally provided on the freesurface of the phosphor layer (surface not facing the support) to keepthe phosphor layer from chemical deterioration or physical shock.

In other words, the radiation image recording and reproducing method ispreferably carried out using the radiation image storage panelcomprising a phosphor layer containing the divalent europium activatedalkaline earth metal halide phosphor coactivated with boron and havingthe above formula (I).

In the method utilizing the stimulable phosphor having the formula (I)in the form of the panel, a radiation having passed through an object orradiated from an object is absorbed by the phosphor layer of the panelto form a radiation image as a radiation energy-stored image on thepanel. The panel is then excited (e.g., scanned) with an electromagneticwave in the wavelength region of 450-1000 nm to release the stored imageas stimulated emission. The emitted light is photoelectrically detectedto obtain electric signals so that the radiation image of the object canbe reproduced as a visible image from the electric signals.

The radiation image recording and reproducing method of the presentinvention will be described in more detail with respect to an example ofa radiation image storage panel containing the stimulable phosphorhaving the formula (I), by referring to a schematic view shown in FIG.2.

In FIG. 2 which shows the total system of the radiation image recordingand reproducing method of the invention, a radiation generating device11 such as an X-ray source provides a radiation for irradiating anobject 12 therewith; a radiation image storage panel 13 containing thestimulable phosphor having the formula (I) absorbs and stores theradiation having passed through the object 12; a source of stimulatingrays 14 provides an electromagnetic wave for releasing the radiationenergy stored in the panel 13 as light emission; a photosensor 15 suchas a photomultiplier faces the panel 13 for detecting the light emittedby the panel 13 and converting it to electric signals; an imagereproducing device 16 is connected with the photosensor 15 to reproducea radiation image from the electric signals detected by the photosensor15; a display device 17 is connected with the reproducing device 16 todisplay the reproduced image in the form of a visible image on a CRT orthe like; and a filter 18 is disposed in front of the photosensor 15 tocut off the stimulating rays reflected by the panel 13 and allow onlythe light emitted by the panel 13 to pass through.

FIG. 2 illustrates an example of the system according to the method ofthe invention employed for obtaining a radiation-transmission image ofan object. However, in the case that the object 12 itself emits aradiation, it is unnecessary to install the above-mentioned radiationgenerating device 11. Further, the photosensor 15 to the display device17 in the system can be replaced with other appropriate devices whichcan reproduce a radiation image having the information of the object 12from the light emitted by the panel 13.

Referring to FIG. 2, when the object 12 is exposed to a radiation suchas X-rays provided by the radiation generating device 11, the radiationpasses through the object 12 in proportion to the radiationtransmittance of each portion of the object. The radiation having passedthrough the object 12 impinges upon the radiation image storage panel13, and is absorbed by the phosphor layer of the panel 13. Thus, aradiation energy-stored image (a kind of latent image) corresponding tothe radiation-transmission image of the object 12 is formed on the panel13.

Thereafter, when the radiation image storage panel 13 is irradiated withan electromagnetic wave having the wavelength within the range of450-1000 nm, which is provided by the source of stimulating rays 14, theradiation energy-stored image formed on the panel 13 is released aslight emission. The intensity of so released light is in proportion tothe intensity of the radiation energy which has been absorbed by thephosphor layer of the panel 13. The light signals corresponding to theintensity of the emitted light are converted to electric signals bymeans of the photosensor 15, the electric signals are reproduced as animage in the image reproducing device 16, and the reproduced image isdisplayed on the display device 17.

The detection of the radiation image stored in the panel 13 can be, forexample, carried out by scanning the panel with the electromagnetic wavesuch as a laser beam provided by the source of stimulating rays 14 anddetecting the light emitted from the panel 13 under scanning by means ofthe photosensor 15 such as photomultiplier to sequentially obtainelectric signals.

In the radiation image recording and reproducing method of the presentinvention, there is no specific limitation on the radiation employablefor exposure of an object to obtain a radiation-transmittance imagethereof, as far as the above-described phosphor gives stimulatedemission upon excitation with the electromagnetic wave after exposure tothe radiation. Examples of the radiation employable in the inventioninclude those generally known, such as X-rays, cathode rays andultraviolet rays. Likewise, there is no specific limitation on theradiation radiating from an object for obtaining a radiation imagethereof, as far as the radiation can be absorbed by the above-describedphosphor to serve as an energy source for producing the stimulatedemission. Examples of the radiation include γ-rays, α-rays and β-rays.

As the source of stimulating rays for exciting the phosphor which hasabsorbed the radiation having passed through or radiated from theobject, there can be employed, for instance, light sources providinglight having a band spectrum distribution in the wavelength region of450-1000 nm; and light sources providing light having a singlewavelength or more in said region such as an Ar ion laser, a Kr ionlaser, a He-Ne laser, a ruby laser, a semiconductor laser, a glasslaser, a YAG laser, a dye laser and a light emitting diode (LED). Amongthe above-mentioned sources of stimulating rays, the lasers arepreferred because the radiation image storage panel is exposed theretowith a high energy density per unit area. Particularly preferred are aHe-Ne laser, Ar ion laser and Kr ion laser from the viewpoints ofstability, output power, etc. The semiconductor laser is also preferred,because its size is small, it can be driven by a weak electric power andits output power can be easily stabilized because of the directmodulation thereof.

The radiation image storage panel employable in the radiation imagerecording and reproducing method of the invention will be described.

The radiation image storage panel, as described hereinbefore,essentially comprises a support and a phosphor layer provided thereonwhich comprises a binder and the above-described divalent europiumactivated alkaline earth metal halide phosphor coactivated with boronand having the formula (I) dispersed therein.

The radiation image storage panel having such structure can be prepared,for instance, in the manner described below.

Examples of the binder to be employed in the phosphor layer include:natural polymers such as proteins (e.g. gelatin), polysaccharides (e.g.dextran) and gum arabic; and synthetic polymers such as polyvinylbutyral, polyvinyl acetate, nitrocellulose, ethylcellulose, vinylidenechloride-vinyl chloride copolymer, polyalkyl (meth)acrylate, vinylchloride-vinyl acetate copoymer, polyurethane, cellulose acetatebutyrate, polyvinyl alcohol, and linear polyester. Particularlypreferred are nitrocellulose, linear polyester, polyalkyl(meth)acrylate, a mixture of nitrocellulose and linear polyester, and amixture of nitrocellulose and polyalkyl (meth)acrylate.

The phosphor layer can be formed on a support, for instance, by thefollowing procedure.

In the first place, the stimulable phosphor particles and a binder areadded to an appropriate solvent, and then they are thoroghly mixed toprepare a coating dispersion of the phosphor particles in the bindersolution.

Examples of the solvent employable in the preparation of the coatingdispersion include lower alcohols such as methanol, ethanol, n-propanoland n-butanol; chlorinated hydrocarbons such as methylene chloride andethylene chloride; ketones such as acetone, methyl ethyl ketone andmethyl isobutyl ketone; esters of lower alcohols with lower aliphaticacids such as methyl acetate, ethyl acetate and butyl acetate; etherssuch as dioxane, ethylene glycol monoethylether and ethylene glycolmonoethyl ether; and mixtures of the above-mentioned compounds.

The ratio between the binder and the phosphor in the coating dispersionmay be determined according to the characteristics of the aimedradiation image storage panel and the nature of the phosphor employed.Generally, the ratio therebetween is within the range of from 1:1 to1:100 (binder : phosphor, by weight), preferably from 1:8 to 1:40.

The coating dispersion may contain a dispersing agent to assist thedispersibility of the phosphor particles therein, and also contain avariety of additives such as a plasticizer for increasing the bondingbetween the binder and the phosphor particles in the phosphor layer.Examples of the dispersing agent include phthalic acid, stearic acid,caproic acid and a hydrophobic surface active agent. Examples of theplasticizer include phosphates such as triphenyl phosphate, tricresylphosphate and diphenyl phosphate; phthalates such as diethyl phthalateand dimethoxyethyl phthalate; glycolates such as ethylphthalyl ethylglycolate and butylphthalyl butyl glycolate; and polyesters ofpolyethylene glycols with aliphatic dicarboxylic acids such as polyesterof triethylene glycol with adipic acid and polyester of diethyleneglycol with succinic acid.

The coating dispersion containing the phosphor particles and the binderprepared as described above is applied evenly to the surface of asupport to form a layer of the coating dispersion. The coating procedurecan be carried out by a conventional method such as a method using adoctor blade, a roll coater or a knife coater.

A support material employed in the present invention can be selectedfrom those employed in the conventional radiogaphic intensifying screensor those employed in the known radiation image storage panels. Examplesof the support material include plastic films such as films of celluloseacetate, polyester, polyethylene terephthalate, polyamide, polyimide,triacetate and polycarbonate; metal sheets such as aluminum foil andaluminium alloy foil; ordinary papers; baryta paper; resin-coatedpapers; pigment papers containing titanium dioxide or the like; andpapers sized with polyvinyl alcohol or the like. From the viewpoint ofcharacteristics of a radiation image storage panel as an informationrecording material, a plastic film is preferably employed as the supportmaterial of the invention. The plastic film may contain alight-absorbing material such as carbon black, or may contain alight-reflecting material such as titanium dioxide. The former isappropriate for preparing a high-sharpness type radiation image storagepanel, while the latter is appropriate for preparing a high-sensitivitytype radiation image storage panel.

In the preparation of a known radiation image storage panel, one or moreadditional layers are occasionally provided between the support and thephosphor layer, so as to enhance the adhesion between the support andthe phosphor layer, or to improve the sensitivity of the panel or thequality of an image (sharpness and graininess) provided thereby. Forinstance, a subbing layer or an adhesive layer may be provided bycoating a polymer material such as gelatin over the surface of thesupport on the phosphor layer side. Otherwise, a light-reflecting layeror a light-absorbing layer may be provided by forming a polymer materiallayer containing a light-reflecting material such as titanium dioxide ora light-absorbing material such as carbon black. In the invention, oneor more of these additional layers may be provided, and the constitutionthereof can be optionally selected depending upon the purpose of theradiation image storage panel.

As described in U.S. patent application Ser. No. 496,278 (the wholecontent of which is described in European Patent Publication No. 92241),the phosphor layer-side surface of the support (or the surface of anadhesive layer, light-reflecting layer, or light-absorbing layer in thecase that such layers are provided on the phosphor layer) may beprovided with protruded and depressed portions for enhancement of thesharpness of radiation image.

After applying the coating dispersion to the support as descrived above,the coating dispersion is then heated slowly to dryness so as tocomplete the formation of a phosphor layer. The thickness of thephosphor layer varies depending upon the characteristics of the aimedradiation image storage panel, the nature of the phosphor, the ratiobetween the binder and the phosphor, etc. Generally, the thickness ofthe phosphor layer is within the range of from 20 μm to 1 mm, preferablyfrom 50 to 500 μm.

The phosphor layer can be provided on the support by the methods otherthan that given in the above. For instance, the phosphor layer isinitially prepared on a sheet (false support) such as a glass plate,metal plate or plastic sheet using the aforementioned coating dispersionand then thus prepared phosphor layer is overlaid on the genuine supportby pressing or using an adhesive agent.

The phosphor layer placed on the support can be in the form of a singlelayer or in the form of plural (two or more) layers. When the pluralphosphor layrers are placed, at least one layer contains theaforementioned divalent europium activated alakline earth metal halidephosphor coactivated with boron and having the formula (I), and theplural layers may be placed in such a manner that a layer nearer to thesurface shows stimulated emission of higher intensity. In any case, thatis, in either the single phosphor layer or plural phosphor layers, avariety of known stimulable phosphors are employable in combination withthe above-mentioned stimulable phosphor.

Examples of the stimulable phosphor employable in combination with thestimulable phosphor of the invention include the aforementioned phosphorand the phosphors described below;

ZnS:Cu,Pb, BaO.xAl₂ O₃ :Eu, in which x is a number satisfying thecondition of 0.8≦x≦10, and M^(II) O.xSiO₂ :A, in which M^(II) is atleast one divalent metal selected from the group consisting of Mg, Ca,Sr, Zn, Cd and Ba, A is at least one element selected from the groupconsisting of Ce, Tb, Eu, Tm, Pb, Tl, Bi and Mn, and x is a numbersatisfying the condition of 0.5≦x≦2.5, as described in U.S. Pat. No.4,326,078;

(Ba_(1-x-y), Mg_(x), Ca_(y))FX:aEu²⁺, in which X is at least one elementselected from the group consisting of Cl and Br, x and y are numberssatisfying the conditions of 0<x+y≦0.6, and xy≠0, and a is a numbersatisfying the condition of 10⁻⁶ ≦a≦5×10⁻², as described in JapanesePatent Provisional Publication No. 55(1980)-12143; and

LnOX:xA, in which Ln is at least one element selected from the groupconsisting of La, Y, Gd and Lu, X is at least one element selected fromthe group consisting of Cl and Br, A is at least one element selectedfrom the group consisting of Ce and Tb, and x is a number satisfying thecondition of 0<x<0.1, as described in the above-mentioned U.S. Pat. No.4,236,078.

A radiation image storage panel generally has a transparent film on afree surface of a phosphor layer to physically and chemically protectthe phosphor layer. In the panel of the present invention, it ispreferable to provide a transparent film for the same purpose.

The transparent film can be provided on the phosphor layer by coatingthe surface of the phosphor layer with a solution of a transparentpolymer such as a cellulose derivative (e.g. cellulose acetate ornitrocellulose), or a synthetic polymer (e.g. polymethyl methacrylate,polyvinyl butyral, polyvinyl formal, polycarbonate, polyvinyl acetate,or vinyl chloride-vinyl acetate copolymer), and drying the coatedsolution. Alternatively, the transparent film can be provided on thephosphor layer by beforehand preparing it from a polymer such aspolyethylene terephthalate, polyethylene, polyvinylidene chloride orpolyamide, followed by placing and fixing it onto the phosphor layerwith an appropriate adhesive agent. The transparent protective filmpreferably has a thickness within the range of approximately 0.1 to 20μm.

The present invention will be illustrated by the following examples, butthese examples by no means restrict the invention.

EXAMPLE 1

To 800 ml of distilled water (H₂ O) were added 333.2 g. of bariumbromide (BaBr₂.2H₂ O), 244.3 g. of barium chloride (BaCl₂.2H₂ O), 0.0696g. of boron oxide (B₂ O₃) and 0.783 g. of europium bromide (EuBr₃), andthey were mixed to obtain an aqueous solution. The aqueous solution wasdried at 60° C. under reduced pressure for 3 hours and further dried at150° C. under vacuum for another 3 hours to obtain a mixture of thestarting materials for the preparation of a phosphor.

The mixture was placed in an alumina crucible, which was, in turn,placed in a high-temperature electric furnace. The mixture was thenfired at 900° C. for 1.5 hours in a carbon dioxide atmosphere containingcarbon monoxide. After the firing was complete, the crucible was takenout of the furnace and allowed to stand for cooling. Thus, a powderydivalent europium activated barium chlorobromide phosphor which iscoactivated with boron (BaCl₂.BaBr₂ :0.001Eu²⁺,0.002B) was obtained.

Further, the amount of boron was varied within a range of 0-2.0 mols per1 mol of BaCl₂.BaBr₂, to obtain a variety of powdery divalent europiumactivated barium chlorobromide phosphors coactivated with a differentamount of boron (BaCl₂.BaBr₂ :0.001Eu²⁺, yB).

The phosphors prepared in Example 1 were excited with a semiconductorlaser beam (wavelength: 780 nm) after exposure to X-rays at 80 KVp, tomeasure the luminance of stimulated emission. The results are shown inFIG. 1.

FIG. 1 graphically shows a relationship between an amount of boron (yvalue) and luminance of stimulated emission with respect to BaCl₂.BaBr₂:0.001Eu²⁺,yB phosphor.

As is evident from FIG. 1, the BaCl₂.BaBr₂ :0.001Eu²⁺,yB phosphor of theinvention having y value within a range of 2×10⁻⁴ ≦y≦2×10⁻¹ was enhancedin the luminance of stimulated emission, and particularly the phosphorhaving y value within a range of 4×10⁻⁴ ≦y≦10⁻¹ showed stimulatedemission of high luminance.

EXAMPLE 2

The procedure of Example 1 was repeated except for using 0.210 g. ofammonium borofluoride (NH₄ BF₄) instead of boron oxide to obtain apowdery divalent europium activated barium chlorobromide phosphor whichis coactivated with boron (BaCl₂.BaBr₂ :0.001Eu²⁺, 0.002B).

COMPARISON EXAMPLE 1

The procedure of Example 1 was repeated except for adding no boron oxideto obtain a powdery divalent europium activated barium chlorobromidephosphor (BaCl₂.BaBr₂ :0.001Eu²⁺).

The phosphors prepared in Example 2 and Comparison Example 1 wereexcited with a semiconductor laser beam (wavelength: 780 nm) afterexposure to X-rays at 80 KVp, to evaluate the luminance of stimulatedemission.

The results on the evaluation of the phosphors are set forth in Table 1,wherein the result of Example 1 is also set forth.

                  TABLE 1                                                         ______________________________________                                                    Boron    Relative Luminance of                                                Compound Stimulated Emission                                      ______________________________________                                        Example    1      B.sub.2 O.sub.3                                                                          116                                                         2      NH.sub.4 BF.sub.4                                                                        144                                              Com. Example                                                                             1      none       100                                              ______________________________________                                    

EXAMPLE 3

To a mixture of the powdery divalent europium activated bariumchlorobromide phosphor coactivated with boron (BaCl₂.BaBr₂:0.001Eu²⁺,0.002B) which was obtained in Example 1 and a linearpolyester resin were added successively methyl ethyl ketone andnitrocellulose (nitrification degree: 11.5%), to prepare a dispersioncontaining the phosphor and the binder (10:1, by weight). Subsequently,tricresyl phosphate, n-butanol and methyl ethyl ketone were added to thedispersion. The mixture was sufficiently stirred by means of a propelleragitater to obtain a homogeneous coating dispersion having a viscosityof 25-35 PS (at 25° C.).

The coating dispersion was applied to a polyethylene terephthalate sheetcontaining titanium dioxide (support, thickness: 250 μm) placedhorizontally on a glass plate. The application of the coating dispersionwas carried out using a doctor blade. The support having a layer of thecoating dispersion was then placed in an oven and heated at atemperature gradually rising from 25° to 100° C. Thus, a phosphor layerhaving a thickness of 250 μm was formed on the support.

On the phosphor layer was placed a transparent polyethyleneterephthalate film (thickness: 12 μm; provided with a polyester adhesivelayer on one surface) to combine the transparent film and the phosphorlayer with the adhesive layer.

Thus, a radiation image storage panel consisting essentially of asupport, a phosphor layer and a transparent protective film wasprepared.

EXAMPLE 4

The procedure of Example 3 was repeated except for employing theBaCl₂.BaBr₂ :0.001Eu²⁺, 0.002B phosphor obtained in Example 2 instead ofthe phosphor obtained in Example 1, to prepare a radiation image storagepanel consisting essentially of a support, a phosphor layer and atransparent protective film.

COMPARISON EXAMPLE 2

The procedure of Example 3 was repeated except for employing theBaCl₂.BaBr₂ :0.001Eu²⁺ phosphor obtained in Comparison Example 1 insteadof the BaCl₂.BaBr₂ : 0.001Eu²⁺,0.002B phosphor, to prepare a radiationimage storage panel consisting essentially of a support, a phosphorlayer and a transparent protective film.

The radiation image storage panels prepared in Examples 3 and 4 andComparison Example 2 were measured on the sensitivity (i.e., luminanceof stimulated emission) when excited with a semiconductor laser beam(wavelength: 780 nm) after exposure to X-rays at 80 KVp.

The results on the evaluation of the panels are set forth in Table 2.

                  TABLE 2                                                         ______________________________________                                                      Boron   Relative                                                              Compound                                                                              Sensitivity                                             ______________________________________                                        Example     3       B.sub.2 O.sub.3                                                                         115                                                         4       NH.sub.4 BF.sub.4                                                                       140                                             Com. Example                                                                              2       none      100                                             ______________________________________                                    

We claim:
 1. A divalent europium activated alkaline earth metal halidephosphor which is coactivated with boron and has the formula (I):

    M.sup.II X.sub.2.aM.sup.II X'.sub.2 :xEu.sup.2+,yB         (I)

in which M^(II) is at least one alkaline earth metal selected from thegroup consisting of Ba, Sr and Ca; each of X and X' is at least onehalogen selected from the group consisting of Cl, Br and I, and X≠X';and a, x and y are numbers satisfying the conditions of 0.1≦a≦10.0,0<x≦0.2 and 2×10⁻⁴ ≦y≦2×10⁻¹, respectively.
 2. The phosphor as claimedin claim 1, in which a in the formula (I) is a number satisfying thecondition of 0.3≦a≦3.3.
 3. The phosphor as claimed in claim 1, in whichy in the formula (I) is a number satisfying the condition of 4×10⁻⁴≦y≦10⁻¹.
 4. The phosphor as claimed in claim 1, in which M^(II) in theformula (I) is Ba.
 5. The phosphor as claimed in claim 1, in which eachof X and X' in the formula (I) is Cl or Br.
 6. The phosphor as claimedin claim 1, in which x in the formula (I) is a number satisfying thecondition of 10⁻⁵ ≦x≦10⁻¹.
 7. A radiation image storage panel comprisinga support and a stimulable phosphor layer provided thereon, in whichsaid stimulable phosphor layer contains a divalent europium activatedalkaline earth metal halide phosphor which is coactivated with boron andhas the formula (I):

    M.sup.II X.sub.2.aM.sup.II X'.sub.2 :xEu.sup.2+,yB         (I)

in which M^(II) is at least one alkaline earth metal selected from thegroup consisting of Ba, Sr and Ca; each of X and X' is at least onehalogen selected from the group consisting of Cl, Br and I, and X≠X';and a, x and y are numbers satisfying the conditions of 0.1≦a≦10.0,0<x≦0.2 and 2×10⁻⁴ ≦y≦2×10⁻¹, respectively.
 8. The radiation imagestorage panel as claimed in claim 7, in which a in the formula (I) is anumber satisfying the condition of 0.3≦a≦3.3.
 9. The radiation imagestorage panel as claimed in claim 7, in which y in the formula (I) is anumber satisfying the condition of 4×10⁻⁴ ≦y≦10⁻¹.
 10. The radiationimage storage panel as claimed in claim 7, in which M^(II) in theformula (I) is Ba.
 11. The radiation image storage panel as claimed inclaim 7, in which each of X and X' in the formula (I) is Cl or Br. 12.The radiation image storage panel as claimed in claim 7, in which x inthe formula (I) is a number satisfying the condition of 10⁻⁵ ≦x≦10⁻¹.